When a Cognitive Radio (CR) system co-exists with a primary system (e.g., a broadcast system), that is, when a CR system shares a frequency band of the primary system, the CR system has to sense the primary system either periodically or according to a pre-defined procedure in order to avoid interference to the primary system. In general, compared with out-band channels in which the CR is not operating, the sensing process is more frequently performed through in-band channels in which the CR system is operating.
According to the Institute of Electrical and Electronics Engineers (IEEE) 802.22 standard, the CR system operates through one TeleVision (TV) channel. As defined in this standard, the in-band channels include not only neighbor channels N+1 and N−1 but also a channel N currently in use.
In order to perform the sensing process through the in-band channels, the CR system has to terminate communication using the sensed in-band channels. A sensing period is referred to as a Quiet Period (QP). Neighbor cells operating through the in-band channels also have to terminate transmission during the QP to improve sensing capability.
Meanwhile, in a case where a plurality of CR systems co-exist, if the plurality of CR systems operate through the same channel or neighbor channels, QP scheduling is necessary to avoid mutual interference. To allow the QP scheduling, QP scheduling information has to be shared between the CR systems. In other words, the QP scheduling information has to be broadcast to the neighbor cells so that the neighbor cells follow a scheduled QP. According to the IEEE 802.22 standard, the QP scheduling information is broadcast by using a Superframe Control Header (SCH) or a Coexistence Beacon Protocol (CBP) packet.
FIGS. 1A, 1B and 1C illustrate an example of QP scheduling according to the prior art.
Referring to FIG. 1A, if it is assumed that cells 1, 2, 3, and 4 respectively use channels N, N+1, N+2, and N+3, the cell 1 schedules and broadcasts a QP for the channel N. In this case, the cell 2 using the channel N+1 performs QP scheduling of the channel N+1 by considering QP scheduling of the cell 1. The channels N+1 and N−1 are in-band channels of the channel N. The in-band channel synchronizes QP scheduling.
Thereafter, the cell 2 using the channel N+1 adjusts timing to the QP scheduling of the cell 1 and broadcast QP scheduling information of the cell 2. In this case, the cell 3 using the channel N+2 performs QP scheduling of the channel N+2 by considering the QP scheduling of the cell 2. The channels N and N+2 are in-band channels of the channel N+1.
Thereafter, the cell 3 using the channel N+2 adjusts timing to the QP scheduling of the cell 2 and broadcast QP scheduling information of the cell 3. In this case, the cell 4 using the channel N+3 regulates QP scheduling of the channel N+3 by considering the QP scheduling of the cell 3. The channels N+1 and N+3 are in-band channels of the channel N+2. Therefore, the QP scheduling of the cell 1 propagates up to the cell 4.
However, the cell 4 does not operate in the in-band channel of the cell 1. That is, although the cell 1 uses the channel N and the cell 4 uses the channel N+3, the cell 4 is synchronized to QP scheduling timing of the cell 1. As a result, QP scheduling of an out-band channel is unnecessarily performed, thereby decreasing overall system throughput.
Referring to FIG. 1B, if it is assumed that cells 1, 2, and 3 respectively use channels N, N−1, and N+1, similarly to FIG. 1A, the cell 2 using the channel N−1 adjusts timing to QP scheduling of the cell 1 and broadcasts QP scheduling information of the cell 2. Since the channel N+1 and the channel N−1 are not in-band channels with each other, the cell 3 using the channel N+1 ignores QP scheduling information of the cell 2 using the channel N−1. However, since the cell 1 using the channel N and the cell 3 using the channel N+1 use in-band channels, QP scheduling has to be synchronized.
Referring to FIG. 1C, if it is assumed that cells 1, 2, and 3 respectively use channel N, M, and N, the cell 2 using the channel M which is an out-band channel of the channel N ignores QP scheduling information from the cell 1 and broadcast QP scheduling information for the channel M. Likewise, the cell 3 ignores QP scheduling information for the channel M. However, since the cell 1 and the cell 3 have to use the same channel N, QP scheduling has to be synchronized.
As described above, a problem arises in the conventional method when QP scheduling synchronization is not provided to neighbor BSs using an in-band channel and when unnecessary QP scheduling synchronization is provided to neighbor BSs using an out-band channel.